UBIFS: add new flash file system

This is a new flash file system. See
http://www.linux-mtd.infradead.org/doc/ubifs.html

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: Adrian Hunter <ext-adrian.hunter@nokia.com>

1 files changed, 1275 insertions, 0 deletions

diff --git a/fs/ubifs/file.c b/fs/ubifs/file.c
new file mode 100644
index 00000000000..005a3b854d9
--- /dev/null
+++ b/fs/ubifs/file.c
@@ -0,0 +1,1275 @@
+/*
+ * This file is part of UBIFS.
+ *
+ * Copyright (C) 2006-2008 Nokia Corporation.
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 as published by
+ * the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along with
+ * this program; if not, write to the Free Software Foundation, Inc., 51
+ * Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
+ *
+ * Authors: Artem Bityutskiy (Битюцкий Артём)
+ *          Adrian Hunter
+ */
+
+/*
+ * This file implements VFS file and inode operations of regular files, device
+ * nodes and symlinks as well as address space operations.
+ *
+ * UBIFS uses 2 page flags: PG_private and PG_checked. PG_private is set if the
+ * page is dirty and is used for budgeting purposes - dirty pages should not be
+ * budgeted. The PG_checked flag is set if full budgeting is required for the
+ * page e.g., when it corresponds to a file hole or it is just beyond the file
+ * size. The budgeting is done in 'ubifs_write_begin()', because it is OK to
+ * fail in this function, and the budget is released in 'ubifs_write_end()'. So
+ * the PG_private and PG_checked flags carry the information about how the page
+ * was budgeted, to make it possible to release the budget properly.
+ *
+ * A thing to keep in mind: inode's 'i_mutex' is locked in most VFS operations
+ * we implement. However, this is not true for '->writepage()', which might be
+ * called with 'i_mutex' unlocked. For example, when pdflush is performing
+ * write-back, it calls 'writepage()' with unlocked 'i_mutex', although the
+ * inode has 'I_LOCK' flag in this case. At "normal" work-paths 'i_mutex' is
+ * locked in '->writepage', e.g. in "sys_write -> alloc_pages -> direct reclaim
+ * path'. So, in '->writepage()' we are only guaranteed that the page is
+ * locked.
+ *
+ * Similarly, 'i_mutex' does not have to be locked in readpage(), e.g.,
+ * readahead path does not have it locked ("sys_read -> generic_file_aio_read
+ * -> ondemand_readahead -> readpage"). In case of readahead, 'I_LOCK' flag is
+ * not set as well. However, UBIFS disables readahead.
+ *
+ * This, for example means that there might be 2 concurrent '->writepage()'
+ * calls for the same inode, but different inode dirty pages.
+ */
+
+#include "ubifs.h"
+#include <linux/mount.h>
+
+static int read_block(struct inode *inode, void *addr, unsigned int block,
+		      struct ubifs_data_node *dn)
+{
+	struct ubifs_info *c = inode->i_sb->s_fs_info;
+	int err, len, out_len;
+	union ubifs_key key;
+	unsigned int dlen;
+
+	data_key_init(c, &key, inode->i_ino, block);
+	err = ubifs_tnc_lookup(c, &key, dn);
+	if (err) {
+		if (err == -ENOENT)
+			/* Not found, so it must be a hole */
+			memset(addr, 0, UBIFS_BLOCK_SIZE);
+		return err;
+	}
+
+	ubifs_assert(dn->ch.sqnum > ubifs_inode(inode)->creat_sqnum);
+
+	len = le32_to_cpu(dn->size);
+	if (len <= 0 || len > UBIFS_BLOCK_SIZE)
+		goto dump;
+
+	dlen = le32_to_cpu(dn->ch.len) - UBIFS_DATA_NODE_SZ;
+	out_len = UBIFS_BLOCK_SIZE;
+	err = ubifs_decompress(&dn->data, dlen, addr, &out_len,
+			       le16_to_cpu(dn->compr_type));
+	if (err || len != out_len)
+		goto dump;
+
+	/*
+	 * Data length can be less than a full block, even for blocks that are
+	 * not the last in the file (e.g., as a result of making a hole and
+	 * appending data). Ensure that the remainder is zeroed out.
+	 */
+	if (len < UBIFS_BLOCK_SIZE)
+		memset(addr + len, 0, UBIFS_BLOCK_SIZE - len);
+
+	return 0;
+
+dump:
+	ubifs_err("bad data node (block %u, inode %lu)",
+		  block, inode->i_ino);
+	dbg_dump_node(c, dn);
+	return -EINVAL;
+}
+
+static int do_readpage(struct page *page)
+{
+	void *addr;
+	int err = 0, i;
+	unsigned int block, beyond;
+	struct ubifs_data_node *dn;
+	struct inode *inode = page->mapping->host;
+	loff_t i_size = i_size_read(inode);
+
+	dbg_gen("ino %lu, pg %lu, i_size %lld, flags %#lx",
+		inode->i_ino, page->index, i_size, page->flags);
+	ubifs_assert(!PageChecked(page));
+	ubifs_assert(!PagePrivate(page));
+
+	addr = kmap(page);
+
+	block = page->index << UBIFS_BLOCKS_PER_PAGE_SHIFT;
+	beyond = (i_size + UBIFS_BLOCK_SIZE - 1) >> UBIFS_BLOCK_SHIFT;
+	if (block >= beyond) {
+		/* Reading beyond inode */
+		SetPageChecked(page);
+		memset(addr, 0, PAGE_CACHE_SIZE);
+		goto out;
+	}
+
+	dn = kmalloc(UBIFS_MAX_DATA_NODE_SZ, GFP_NOFS);
+	if (!dn) {
+		err = -ENOMEM;
+		goto error;
+	}
+
+	i = 0;
+	while (1) {
+		int ret;
+
+		if (block >= beyond) {
+			/* Reading beyond inode */
+			err = -ENOENT;
+			memset(addr, 0, UBIFS_BLOCK_SIZE);
+		} else {
+			ret = read_block(inode, addr, block, dn);
+			if (ret) {
+				err = ret;
+				if (err != -ENOENT)
+					break;
+			}
+		}
+		if (++i >= UBIFS_BLOCKS_PER_PAGE)
+			break;
+		block += 1;
+		addr += UBIFS_BLOCK_SIZE;
+	}
+	if (err) {
+		if (err == -ENOENT) {
+			/* Not found, so it must be a hole */
+			SetPageChecked(page);
+			dbg_gen("hole");
+			goto out_free;
+		}
+		ubifs_err("cannot read page %lu of inode %lu, error %d",
+			  page->index, inode->i_ino, err);
+		goto error;
+	}
+
+out_free:
+	kfree(dn);
+out:
+	SetPageUptodate(page);
+	ClearPageError(page);
+	flush_dcache_page(page);
+	kunmap(page);
+	return 0;
+
+error:
+	kfree(dn);
+	ClearPageUptodate(page);
+	SetPageError(page);
+	flush_dcache_page(page);
+	kunmap(page);
+	return err;
+}
+
+/**
+ * release_new_page_budget - release budget of a new page.
+ * @c: UBIFS file-system description object
+ *
+ * This is a helper function which releases budget corresponding to the budget
+ * of one new page of data.
+ */
+static void release_new_page_budget(struct ubifs_info *c)
+{
+	struct ubifs_budget_req req = { .recalculate = 1, .new_page = 1 };
+
+	ubifs_release_budget(c, &req);
+}
+
+/**
+ * release_existing_page_budget - release budget of an existing page.
+ * @c: UBIFS file-system description object
+ *
+ * This is a helper function which releases budget corresponding to the budget
+ * of changing one one page of data which already exists on the flash media.
+ */
+static void release_existing_page_budget(struct ubifs_info *c)
+{
+	struct ubifs_budget_req req = { .dd_growth = c->page_budget};
+
+	ubifs_release_budget(c, &req);
+}
+
+static int write_begin_slow(struct address_space *mapping,
+			    loff_t pos, unsigned len, struct page **pagep)
+{
+	struct inode *inode = mapping->host;
+	struct ubifs_info *c = inode->i_sb->s_fs_info;
+	pgoff_t index = pos >> PAGE_CACHE_SHIFT;
+	struct ubifs_budget_req req = { .new_page = 1 };
+	int uninitialized_var(err), appending = !!(pos + len > inode->i_size);
+	struct page *page;
+
+	dbg_gen("ino %lu, pos %llu, len %u, i_size %lld",
+		inode->i_ino, pos, len, inode->i_size);
+
+	/*
+	 * At the slow path we have to budget before locking the page, because
+	 * budgeting may force write-back, which would wait on locked pages and
+	 * deadlock if we had the page locked. At this point we do not know
+	 * anything about the page, so assume that this is a new page which is
+	 * written to a hole. This corresponds to largest budget. Later the
+	 * budget will be amended if this is not true.
+	 */
+	if (appending)
+		/* We are appending data, budget for inode change */
+		req.dirtied_ino = 1;
+
+	err = ubifs_budget_space(c, &req);
+	if (unlikely(err))
+		return err;
+
+	page = __grab_cache_page(mapping, index);
+	if (unlikely(!page)) {
+		ubifs_release_budget(c, &req);
+		return -ENOMEM;
+	}
+
+	if (!PageUptodate(page)) {
+		if (!(pos & PAGE_CACHE_MASK) && len == PAGE_CACHE_SIZE)
+			SetPageChecked(page);
+		else {
+			err = do_readpage(page);
+			if (err) {
+				unlock_page(page);
+				page_cache_release(page);
+				return err;
+			}
+		}
+
+		SetPageUptodate(page);
+		ClearPageError(page);
+	}
+
+	if (PagePrivate(page))
+		/*
+		 * The page is dirty, which means it was budgeted twice:
+		 *   o first time the budget was allocated by the task which
+		 *     made the page dirty and set the PG_private flag;
+		 *   o and then we budgeted for it for the second time at the
+		 *     very beginning of this function.
+		 *
+		 * So what we have to do is to release the page budget we
+		 * allocated.
+		 */
+		release_new_page_budget(c);
+	else if (!PageChecked(page))
+		/*
+		 * We are changing a page which already exists on the media.
+		 * This means that changing the page does not make the amount
+		 * of indexing information larger, and this part of the budget
+		 * which we have already acquired may be released.
+		 */
+		ubifs_convert_page_budget(c);
+
+	if (appending) {
+		struct ubifs_inode *ui = ubifs_inode(inode);
+
+		/*
+		 * 'ubifs_write_end()' is optimized from the fast-path part of
+		 * 'ubifs_write_begin()' and expects the @ui_mutex to be locked
+		 * if data is appended.
+		 */
+		mutex_lock(&ui->ui_mutex);
+		if (ui->dirty)
+			/*
+			 * The inode is dirty already, so we may free the
+			 * budget we allocated.
+			 */
+			ubifs_release_dirty_inode_budget(c, ui);
+	}
+
+	*pagep = page;
+	return 0;
+}
+
+/**
+ * allocate_budget - allocate budget for 'ubifs_write_begin()'.
+ * @c: UBIFS file-system description object
+ * @page: page to allocate budget for
+ * @ui: UBIFS inode object the page belongs to
+ * @appending: non-zero if the page is appended
+ *
+ * This is a helper function for 'ubifs_write_begin()' which allocates budget
+ * for the operation. The budget is allocated differently depending on whether
+ * this is appending, whether the page is dirty or not, and so on. This
+ * function leaves the @ui->ui_mutex locked in case of appending. Returns zero
+ * in case of success and %-ENOSPC in case of failure.
+ */
+static int allocate_budget(struct ubifs_info *c, struct page *page,
+			   struct ubifs_inode *ui, int appending)
+{
+	struct ubifs_budget_req req = { .fast = 1 };
+
+	if (PagePrivate(page)) {
+		if (!appending)
+			/*
+			 * The page is dirty and we are not appending, which
+			 * means no budget is needed at all.
+			 */
+			return 0;
+
+		mutex_lock(&ui->ui_mutex);
+		if (ui->dirty)
+			/*
+			 * The page is dirty and we are appending, so the inode
+			 * has to be marked as dirty. However, it is already
+			 * dirty, so we do not need any budget. We may return,
+			 * but @ui->ui_mutex hast to be left locked because we
+			 * should prevent write-back from flushing the inode
+			 * and freeing the budget. The lock will be released in
+			 * 'ubifs_write_end()'.
+			 */
+			return 0;
+
+		/*
+		 * The page is dirty, we are appending, the inode is clean, so
+		 * we need to budget the inode change.
+		 */
+		req.dirtied_ino = 1;
+	} else {
+		if (PageChecked(page))
+			/*
+			 * The page corresponds to a hole and does not
+			 * exist on the media. So changing it makes
+			 * make the amount of indexing information
+			 * larger, and we have to budget for a new
+			 * page.
+			 */
+			req.new_page = 1;
+		else
+			/*
+			 * Not a hole, the change will not add any new
+			 * indexing information, budget for page
+			 * change.
+			 */
+			req.dirtied_page = 1;
+
+		if (appending) {
+			mutex_lock(&ui->ui_mutex);
+			if (!ui->dirty)
+				/*
+				 * The inode is clean but we will have to mark
+				 * it as dirty because we are appending. This
+				 * needs a budget.
+				 */
+				req.dirtied_ino = 1;
+		}
+	}
+
+	return ubifs_budget_space(c, &req);
+}
+
+/*
+ * This function is called when a page of data is going to be written. Since
+ * the page of data will not necessarily go to the flash straight away, UBIFS
+ * has to reserve space on the media for it, which is done by means of
+ * budgeting.
+ *
+ * This is the hot-path of the file-system and we are trying to optimize it as
+ * much as possible. For this reasons it is split on 2 parts - slow and fast.
+ *
+ * There many budgeting cases:
+ *     o a new page is appended - we have to budget for a new page and for
+ *       changing the inode; however, if the inode is already dirty, there is
+ *       no need to budget for it;
+ *     o an existing clean page is changed - we have budget for it; if the page
+ *       does not exist on the media (a hole), we have to budget for a new
+ *       page; otherwise, we may budget for changing an existing page; the
+ *       difference between these cases is that changing an existing page does
+ *       not introduce anything new to the FS indexing information, so it does
+ *       not grow, and smaller budget is acquired in this case;
+ *     o an existing dirty page is changed - no need to budget at all, because
+ *       the page budget has been acquired by earlier, when the page has been
+ *       marked dirty.
+ *
+ * UBIFS budgeting sub-system may force write-back if it thinks there is no
+ * space to reserve. This imposes some locking restrictions and makes it
+ * impossible to take into account the above cases, and makes it impossible to
+ * optimize budgeting.
+ *
+ * The solution for this is that the fast path of 'ubifs_write_begin()' assumes
+ * there is a plenty of flash space and the budget will be acquired quickly,
+ * without forcing write-back. The slow path does not make this assumption.
+ */
+static int ubifs_write_begin(struct file *file, struct address_space *mapping,
+			     loff_t pos, unsigned len, unsigned flags,
+			     struct page **pagep, void **fsdata)
+{
+	struct inode *inode = mapping->host;
+	struct ubifs_info *c = inode->i_sb->s_fs_info;
+	struct ubifs_inode *ui = ubifs_inode(inode);
+	pgoff_t index = pos >> PAGE_CACHE_SHIFT;
+	int uninitialized_var(err), appending = !!(pos + len > inode->i_size);
+	struct page *page;
+
+
+	ubifs_assert(ubifs_inode(inode)->ui_size == inode->i_size);
+
+	if (unlikely(c->ro_media))
+		return -EROFS;
+
+	/* Try out the fast-path part first */
+	page = __grab_cache_page(mapping, index);
+	if (unlikely(!page))
+		return -ENOMEM;
+
+	if (!PageUptodate(page)) {
+		/* The page is not loaded from the flash */
+		if (!(pos & PAGE_CACHE_MASK) && len == PAGE_CACHE_SIZE)
+			/*
+			 * We change whole page so no need to load it. But we
+			 * have to set the @PG_checked flag to make the further
+			 * code the page is new. This might be not true, but it
+			 * is better to budget more that to read the page from
+			 * the media.
+			 */
+			SetPageChecked(page);
+		else {
+			err = do_readpage(page);
+			if (err) {
+				unlock_page(page);
+				page_cache_release(page);
+				return err;
+			}
+		}
+
+		SetPageUptodate(page);
+		ClearPageError(page);
+	}
+
+	err = allocate_budget(c, page, ui, appending);
+	if (unlikely(err)) {
+		ubifs_assert(err == -ENOSPC);
+		/*
+		 * Budgeting failed which means it would have to force
+		 * write-back but didn't, because we set the @fast flag in the
+		 * request. Write-back cannot be done now, while we have the
+		 * page locked, because it would deadlock. Unlock and free
+		 * everything and fall-back to slow-path.
+		 */
+		if (appending) {
+			ubifs_assert(mutex_is_locked(&ui->ui_mutex));
+			mutex_unlock(&ui->ui_mutex);
+		}
+		unlock_page(page);
+		page_cache_release(page);
+
+		return write_begin_slow(mapping, pos, len, pagep);
+	}
+
+	/*
+	 * Whee, we aquired budgeting quickly - without involving
+	 * garbage-collection, committing or forceing write-back. We return
+	 * with @ui->ui_mutex locked if we are appending pages, and unlocked
+	 * otherwise. This is an optimization (slightly hacky though).
+	 */
+	*pagep = page;
+	return 0;
+
+}
+
+/**
+ * cancel_budget - cancel budget.
+ * @c: UBIFS file-system description object
+ * @page: page to cancel budget for
+ * @ui: UBIFS inode object the page belongs to
+ * @appending: non-zero if the page is appended
+ *
+ * This is a helper function for a page write operation. It unlocks the
+ * @ui->ui_mutex in case of appending.
+ */
+static void cancel_budget(struct ubifs_info *c, struct page *page,
+			  struct ubifs_inode *ui, int appending)
+{
+	if (appending) {
+		if (!ui->dirty)
+			ubifs_release_dirty_inode_budget(c, ui);
+		mutex_unlock(&ui->ui_mutex);
+	}
+	if (!PagePrivate(page)) {
+		if (PageChecked(page))
+			release_new_page_budget(c);
+		else
+			release_existing_page_budget(c);
+	}
+}
+
+static int ubifs_write_end(struct file *file, struct address_space *mapping,
+			   loff_t pos, unsigned len, unsigned copied,
+			   struct page *page, void *fsdata)
+{
+	struct inode *inode = mapping->host;
+	struct ubifs_inode *ui = ubifs_inode(inode);
+	struct ubifs_info *c = inode->i_sb->s_fs_info;
+	loff_t end_pos = pos + len;
+	int appending = !!(end_pos > inode->i_size);
+
+	dbg_gen("ino %lu, pos %llu, pg %lu, len %u, copied %d, i_size %lld",
+		inode->i_ino, pos, page->index, len, copied, inode->i_size);
+
+	if (unlikely(copied < len && len == PAGE_CACHE_SIZE)) {
+		/*
+		 * VFS copied less data to the page that it intended and
+		 * declared in its '->write_begin()' call via the @len
+		 * argument. If the page was not up-to-date, and @len was
+		 * @PAGE_CACHE_SIZE, the 'ubifs_write_begin()' function did
+		 * not load it from the media (for optimization reasons). This
+		 * means that part of the page contains garbage. So read the
+		 * page now.
+		 */
+		dbg_gen("copied %d instead of %d, read page and repeat",
+			copied, len);
+		cancel_budget(c, page, ui, appending);
+
+		/*
+		 * Return 0 to force VFS to repeat the whole operation, or the
+		 * error code if 'do_readpage()' failes.
+		 */
+		copied = do_readpage(page);
+		goto out;
+	}
+
+	if (!PagePrivate(page)) {
+		SetPagePrivate(page);
+		atomic_long_inc(&c->dirty_pg_cnt);
+		__set_page_dirty_nobuffers(page);
+	}
+
+	if (appending) {
+		i_size_write(inode, end_pos);
+		ui->ui_size = end_pos;
+		/*
+		 * Note, we do not set @I_DIRTY_PAGES (which means that the
+		 * inode has dirty pages), this has been done in
+		 * '__set_page_dirty_nobuffers()'.
+		 */
+		__mark_inode_dirty(inode, I_DIRTY_DATASYNC);
+		ubifs_assert(mutex_is_locked(&ui->ui_mutex));
+		mutex_unlock(&ui->ui_mutex);
+	}
+
+out:
+	unlock_page(page);
+	page_cache_release(page);
+	return copied;
+}
+
+static int ubifs_readpage(struct file *file, struct page *page)
+{
+	do_readpage(page);
+	unlock_page(page);
+	return 0;
+}
+
+static int do_writepage(struct page *page, int len)
+{
+	int err = 0, i, blen;
+	unsigned int block;
+	void *addr;
+	union ubifs_key key;
+	struct inode *inode = page->mapping->host;
+	struct ubifs_info *c = inode->i_sb->s_fs_info;
+
+#ifdef UBIFS_DEBUG
+	spin_lock(&ui->ui_lock);
+	ubifs_assert(page->index <= ui->synced_i_size << PAGE_CACHE_SIZE);
+	spin_unlock(&ui->ui_lock);
+#endif
+
+	/* Update radix tree tags */
+	set_page_writeback(page);
+
+	addr = kmap(page);
+	block = page->index << UBIFS_BLOCKS_PER_PAGE_SHIFT;
+	i = 0;
+	while (len) {
+		blen = min_t(int, len, UBIFS_BLOCK_SIZE);
+		data_key_init(c, &key, inode->i_ino, block);
+		err = ubifs_jnl_write_data(c, inode, &key, addr, blen);
+		if (err)
+			break;
+		if (++i >= UBIFS_BLOCKS_PER_PAGE)
+			break;
+		block += 1;
+		addr += blen;
+		len -= blen;
+	}
+	if (err) {
+		SetPageError(page);
+		ubifs_err("cannot write page %lu of inode %lu, error %d",
+			  page->index, inode->i_ino, err);
+		ubifs_ro_mode(c, err);
+	}
+
+	ubifs_assert(PagePrivate(page));
+	if (PageChecked(page))
+		release_new_page_budget(c);
+	else
+		release_existing_page_budget(c);
+
+	atomic_long_dec(&c->dirty_pg_cnt);
+	ClearPagePrivate(page);
+	ClearPageChecked(page);
+
+	kunmap(page);
+	unlock_page(page);
+	end_page_writeback(page);
+	return err;
+}
+
+/*
+ * When writing-back dirty inodes, VFS first writes-back pages belonging to the
+ * inode, then the inode itself. For UBIFS this may cause a problem. Consider a
+ * situation when a we have an inode with size 0, then a megabyte of data is
+ * appended to the inode, then write-back starts and flushes some amount of the
+ * dirty pages, the journal becomes full, commit happens and finishes, and then
+ * an unclean reboot happens. When the file system is mounted next time, the
+ * inode size would still be 0, but there would be many pages which are beyond
+ * the inode size, they would be indexed and consume flash space. Because the
+ * journal has been committed, the replay would not be able to detect this
+ * situation and correct the inode size. This means UBIFS would have to scan
+ * whole index and correct all inode sizes, which is long an unacceptable.
+ *
+ * To prevent situations like this, UBIFS writes pages back only if they are
+ * within last synchronized inode size, i.e. the the size which has been
+ * written to the flash media last time. Otherwise, UBIFS forces inode
+ * write-back, thus making sure the on-flash inode contains current inode size,
+ * and then keeps writing pages back.
+ *
+ * Some locking issues explanation. 'ubifs_writepage()' first is called with
+ * the page locked, and it locks @ui_mutex. However, write-back does take inode
+ * @i_mutex, which means other VFS operations may be run on this inode at the
+ * same time. And the problematic one is truncation to smaller size, from where
+ * we have to call 'vmtruncate()', which first changes @inode->i_size, then
+ * drops the truncated pages. And while dropping the pages, it takes the page
+ * lock. This means that 'do_truncation()' cannot call 'vmtruncate()' with
+ * @ui_mutex locked, because it would deadlock with 'ubifs_writepage()'. This
+ * means that @inode->i_size is changed while @ui_mutex is unlocked.
+ *
+ * But in 'ubifs_writepage()' we have to guarantee that we do not write beyond
+ * inode size. How do we do this if @inode->i_size may became smaller while we
+ * are in the middle of 'ubifs_writepage()'? The UBIFS solution is the
+ * @ui->ui_isize "shadow" field which UBIFS uses instead of @inode->i_size
+ * internally and updates it under @ui_mutex.
+ *
+ * Q: why we do not worry that if we race with truncation, we may end up with a
+ * situation when the inode is truncated while we are in the middle of
+ * 'do_writepage()', so we do write beyond inode size?
+ * A: If we are in the middle of 'do_writepage()', truncation would be locked
+ * on the page lock and it would not write the truncated inode node to the
+ * journal before we have finished.
+ */
+static int ubifs_writepage(struct page *page, struct writeback_control *wbc)
+{
+	struct inode *inode = page->mapping->host;
+	struct ubifs_inode *ui = ubifs_inode(inode);
+	loff_t i_size =  i_size_read(inode), synced_i_size;
+	pgoff_t end_index = i_size >> PAGE_CACHE_SHIFT;
+	int err, len = i_size & (PAGE_CACHE_SIZE - 1);
+	void *kaddr;
+
+	dbg_gen("ino %lu, pg %lu, pg flags %#lx",
+		inode->i_ino, page->index, page->flags);
+	ubifs_assert(PagePrivate(page));
+
+	/* Is the page fully outside @i_size? (truncate in progress) */
+	if (page->index > end_index || (page->index == end_index && !len)) {
+		err = 0;
+		goto out_unlock;
+	}
+
+	spin_lock(&ui->ui_lock);
+	synced_i_size = ui->synced_i_size;
+	spin_unlock(&ui->ui_lock);
+
+	/* Is the page fully inside @i_size? */
+	if (page->index < end_index) {
+		if (page->index >= synced_i_size >> PAGE_CACHE_SHIFT) {
+			err = inode->i_sb->s_op->write_inode(inode, 1);
+			if (err)
+				goto out_unlock;
+			/*
+			 * The inode has been written, but the write-buffer has
+			 * not been synchronized, so in case of an unclean
+			 * reboot we may end up with some pages beyond inode
+			 * size, but they would be in the journal (because
+			 * commit flushes write buffers) and recovery would deal
+			 * with this.
+			 */
+		}
+		return do_writepage(page, PAGE_CACHE_SIZE);
+	}
+
+	/*
+	 * The page straddles @i_size. It must be zeroed out on each and every
+	 * writepage invocation because it may be mmapped. "A file is mapped
+	 * in multiples of the page size. For a file that is not a multiple of
+	 * the page size, the remaining memory is zeroed when mapped, and
+	 * writes to that region are not written out to the file."
+	 */
+	kaddr = kmap_atomic(page, KM_USER0);
+	memset(kaddr + len, 0, PAGE_CACHE_SIZE - len);
+	flush_dcache_page(page);
+	kunmap_atomic(kaddr, KM_USER0);
+
+	if (i_size > synced_i_size) {
+		err = inode->i_sb->s_op->write_inode(inode, 1);
+		if (err)
+			goto out_unlock;
+	}
+
+	return do_writepage(page, len);
+
+out_unlock:
+	unlock_page(page);
+	return err;
+}
+
+/**
+ * do_attr_changes - change inode attributes.
+ * @inode: inode to change attributes for
+ * @attr: describes attributes to change
+ */
+static void do_attr_changes(struct inode *inode, const struct iattr *attr)
+{
+	if (attr->ia_valid & ATTR_UID)
+		inode->i_uid = attr->ia_uid;
+	if (attr->ia_valid & ATTR_GID)
+		inode->i_gid = attr->ia_gid;
+	if (attr->ia_valid & ATTR_ATIME)
+		inode->i_atime = timespec_trunc(attr->ia_atime,
+						inode->i_sb->s_time_gran);
+	if (attr->ia_valid & ATTR_MTIME)
+		inode->i_mtime = timespec_trunc(attr->ia_mtime,
+						inode->i_sb->s_time_gran);
+	if (attr->ia_valid & ATTR_CTIME)
+		inode->i_ctime = timespec_trunc(attr->ia_ctime,
+						inode->i_sb->s_time_gran);
+	if (attr->ia_valid & ATTR_MODE) {
+		umode_t mode = attr->ia_mode;
+
+		if (!in_group_p(inode->i_gid) && !capable(CAP_FSETID))
+			mode &= ~S_ISGID;
+		inode->i_mode = mode;
+	}
+}
+
+/**
+ * do_truncation - truncate an inode.
+ * @c: UBIFS file-system description object
+ * @inode: inode to truncate
+ * @attr: inode attribute changes description
+ *
+ * This function implements VFS '->setattr()' call when the inode is truncated
+ * to a smaller size. Returns zero in case of success and a negative error code
+ * in case of failure.
+ */
+static int do_truncation(struct ubifs_info *c, struct inode *inode,
+			 const struct iattr *attr)
+{
+	int err;
+	struct ubifs_budget_req req;
+	loff_t old_size = inode->i_size, new_size = attr->ia_size;
+	int offset = new_size & (UBIFS_BLOCK_SIZE - 1);
+	struct ubifs_inode *ui = ubifs_inode(inode);
+
+	dbg_gen("ino %lu, size %lld -> %lld", inode->i_ino, old_size, new_size);
+	memset(&req, 0, sizeof(struct ubifs_budget_req));
+
+	/*
+	 * If this is truncation to a smaller size, and we do not truncate on a
+	 * block boundary, budget for changing one data block, because the last
+	 * block will be re-written.
+	 */
+	if (new_size & (UBIFS_BLOCK_SIZE - 1))
+		req.dirtied_page = 1;
+
+	req.dirtied_ino = 1;
+	/* A funny way to budget for truncation node */
+	req.dirtied_ino_d = UBIFS_TRUN_NODE_SZ;
+	err = ubifs_budget_space(c, &req);
+	if (err)
+		return err;
+
+	err = vmtruncate(inode, new_size);
+	if (err)
+		goto out_budg;
+
+	if (offset) {
+		pgoff_t index = new_size >> PAGE_CACHE_SHIFT;
+		struct page *page;
+
+		page = find_lock_page(inode->i_mapping, index);
+		if (page) {
+			if (PageDirty(page)) {
+				/*
+				 * 'ubifs_jnl_truncate()' will try to truncate
+				 * the last data node, but it contains
+				 * out-of-date data because the page is dirty.
+				 * Write the page now, so that
+				 * 'ubifs_jnl_truncate()' will see an already
+				 * truncated (and up to date) data node.
+				 */
+				ubifs_assert(PagePrivate(page));
+
+				clear_page_dirty_for_io(page);
+				if (UBIFS_BLOCKS_PER_PAGE_SHIFT)
+					offset = new_size &
+						 (PAGE_CACHE_SIZE - 1);
+				err = do_writepage(page, offset);
+				page_cache_release(page);
+				if (err)
+					goto out_budg;
+				/*
+				 * We could now tell 'ubifs_jnl_truncate()' not
+				 * to read the last block.
+				 */
+			} else {
+				/*
+				 * We could 'kmap()' the page and pass the data
+				 * to 'ubifs_jnl_truncate()' to save it from
+				 * having to read it.
+				 */
+				unlock_page(page);
+				page_cache_release(page);
+			}
+		}
+	}
+
+	mutex_lock(&ui->ui_mutex);
+	ui->ui_size = inode->i_size;
+	/* Truncation changes inode [mc]time */
+	inode->i_mtime = inode->i_ctime = ubifs_current_time(inode);
+	/* The other attributes may be changed at the same time as well */
+	do_attr_changes(inode, attr);
+
+	err = ubifs_jnl_truncate(c, inode, old_size, new_size);
+	mutex_unlock(&ui->ui_mutex);
+out_budg:
+	ubifs_release_budget(c, &req);
+	return err;
+}
+
+/**
+ * do_setattr - change inode attributes.
+ * @c: UBIFS file-system description object
+ * @inode: inode to change attributes for
+ * @attr: inode attribute changes description
+ *
+ * This function implements VFS '->setattr()' call for all cases except
+ * truncations to smaller size. Returns zero in case of success and a negative
+ * error code in case of failure.
+ */
+static int do_setattr(struct ubifs_info *c, struct inode *inode,
+		      const struct iattr *attr)
+{
+	int err, release;
+	loff_t new_size = attr->ia_size;
+	struct ubifs_inode *ui = ubifs_inode(inode);
+	struct ubifs_budget_req req = { .dirtied_ino = 1,
+					.dirtied_ino_d = ui->data_len };
+
+	err = ubifs_budget_space(c, &req);
+	if (err)
+		return err;
+
+	if (attr->ia_valid & ATTR_SIZE) {
+		dbg_gen("size %lld -> %lld", inode->i_size, new_size);
+		err = vmtruncate(inode, new_size);
+		if (err)
+			goto out;
+	}
+
+	mutex_lock(&ui->ui_mutex);
+	if (attr->ia_valid & ATTR_SIZE) {
+		/* Truncation changes inode [mc]time */
+		inode->i_mtime = inode->i_ctime = ubifs_current_time(inode);
+		/* 'vmtruncate()' changed @i_size, update @ui_size */
+		ui->ui_size = inode->i_size;
+	}
+
+	do_attr_changes(inode, attr);
+
+	release = ui->dirty;
+	if (attr->ia_valid & ATTR_SIZE)
+		/*
+		 * Inode length changed, so we have to make sure
+		 * @I_DIRTY_DATASYNC is set.
+		 */
+		 __mark_inode_dirty(inode, I_DIRTY_SYNC | I_DIRTY_DATASYNC);
+	else
+		mark_inode_dirty_sync(inode);
+	mutex_unlock(&ui->ui_mutex);
+
+	if (release)
+		ubifs_release_budget(c, &req);
+	if (IS_SYNC(inode))
+		err = inode->i_sb->s_op->write_inode(inode, 1);
+	return err;
+
+out:
+	ubifs_release_budget(c, &req);
+	return err;
+}
+
+int ubifs_setattr(struct dentry *dentry, struct iattr *attr)
+{
+	int err;
+	struct inode *inode = dentry->d_inode;
+	struct ubifs_info *c = inode->i_sb->s_fs_info;
+
+	dbg_gen("ino %lu, ia_valid %#x", inode->i_ino, attr->ia_valid);
+	err = inode_change_ok(inode, attr);
+	if (err)
+		return err;
+
+	err = dbg_check_synced_i_size(inode);
+	if (err)
+		return err;
+
+	if ((attr->ia_valid & ATTR_SIZE) && attr->ia_size < inode->i_size)
+		/* Truncation to a smaller size */
+		err = do_truncation(c, inode, attr);
+	else
+		err = do_setattr(c, inode, attr);
+
+	return err;
+}
+
+static void ubifs_invalidatepage(struct page *page, unsigned long offset)
+{
+	struct inode *inode = page->mapping->host;
+	struct ubifs_info *c = inode->i_sb->s_fs_info;
+
+	ubifs_assert(PagePrivate(page));
+	if (offset)
+		/* Partial page remains dirty */
+		return;
+
+	if (PageChecked(page))
+		release_new_page_budget(c);
+	else
+		release_existing_page_budget(c);
+
+	atomic_long_dec(&c->dirty_pg_cnt);
+	ClearPagePrivate(page);
+	ClearPageChecked(page);
+}
+
+static void *ubifs_follow_link(struct dentry *dentry, struct nameidata *nd)
+{
+	struct ubifs_inode *ui = ubifs_inode(dentry->d_inode);
+
+	nd_set_link(nd, ui->data);
+	return NULL;
+}
+
+int ubifs_fsync(struct file *file, struct dentry *dentry, int datasync)
+{
+	struct inode *inode = dentry->d_inode;
+	struct ubifs_info *c = inode->i_sb->s_fs_info;
+	int err;
+
+	dbg_gen("syncing inode %lu", inode->i_ino);
+
+	/*
+	 * VFS has already synchronized dirty pages for this inode. Synchronize
+	 * the inode unless this is a 'datasync()' call.
+	 */
+	if (!datasync || (inode->i_state & I_DIRTY_DATASYNC)) {
+		err = inode->i_sb->s_op->write_inode(inode, 1);
+		if (err)
+			return err;
+	}
+
+	/*
+	 * Nodes related to this inode may still sit in a write-buffer. Flush
+	 * them.
+	 */
+	err = ubifs_sync_wbufs_by_inode(c, inode);
+	if (err)
+		return err;
+
+	return 0;
+}
+
+/**
+ * mctime_update_needed - check if mtime or ctime update is needed.
+ * @inode: the inode to do the check for
+ * @now: current time
+ *
+ * This helper function checks if the inode mtime/ctime should be updated or
+ * not. If current values of the time-stamps are within the UBIFS inode time
+ * granularity, they are not updated. This is an optimization.
+ */
+static inline int mctime_update_needed(const struct inode *inode,
+				       const struct timespec *now)
+{
+	if (!timespec_equal(&inode->i_mtime, now) ||
+	    !timespec_equal(&inode->i_ctime, now))
+		return 1;
+	return 0;
+}
+
+/**
+ * update_ctime - update mtime and ctime of an inode.
+ * @c: UBIFS file-system description object
+ * @inode: inode to update
+ *
+ * This function updates mtime and ctime of the inode if it is not equivalent to
+ * current time. Returns zero in case of success and a negative error code in
+ * case of failure.
+ */
+static int update_mctime(struct ubifs_info *c, struct inode *inode)
+{
+	struct timespec now = ubifs_current_time(inode);
+	struct ubifs_inode *ui = ubifs_inode(inode);
+
+	if (mctime_update_needed(inode, &now)) {
+		int err, release;
+		struct ubifs_budget_req req = { .dirtied_ino = 1,
+						.dirtied_ino_d = ui->data_len };
+
+		err = ubifs_budget_space(c, &req);
+		if (err)
+			return err;
+
+		mutex_lock(&ui->ui_mutex);
+		inode->i_mtime = inode->i_ctime = ubifs_current_time(inode);
+		release = ui->dirty;
+		mark_inode_dirty_sync(inode);
+		mutex_unlock(&ui->ui_mutex);
+		if (release)
+			ubifs_release_budget(c, &req);
+	}
+
+	return 0;
+}
+
+static ssize_t ubifs_aio_write(struct kiocb *iocb, const struct iovec *iov,
+			       unsigned long nr_segs, loff_t pos)
+{
+	int err;
+	ssize_t ret;
+	struct inode *inode = iocb->ki_filp->f_mapping->host;
+	struct ubifs_info *c = inode->i_sb->s_fs_info;
+
+	err = update_mctime(c, inode);
+	if (err)
+		return err;
+
+	ret = generic_file_aio_write(iocb, iov, nr_segs, pos);
+	if (ret < 0)
+		return ret;
+
+	if (ret > 0 && (IS_SYNC(inode) || iocb->ki_filp->f_flags & O_SYNC)) {
+		err = ubifs_sync_wbufs_by_inode(c, inode);
+		if (err)
+			return err;
+	}
+
+	return ret;
+}
+
+static int ubifs_set_page_dirty(struct page *page)
+{
+	int ret;
+
+	ret = __set_page_dirty_nobuffers(page);
+	/*
+	 * An attempt to dirty a page without budgeting for it - should not
+	 * happen.
+	 */
+	ubifs_assert(ret == 0);
+	return ret;
+}
+
+static int ubifs_releasepage(struct page *page, gfp_t unused_gfp_flags)
+{
+	/*
+	 * An attempt to release a dirty page without budgeting for it - should
+	 * not happen.
+	 */
+	if (PageWriteback(page))
+		return 0;
+	ubifs_assert(PagePrivate(page));
+	ubifs_assert(0);
+	ClearPagePrivate(page);
+	ClearPageChecked(page);
+	return 1;
+}
+
+/*
+ * mmap()d file has taken write protection fault and is being made
+ * writable. UBIFS must ensure page is budgeted for.
+ */
+static int ubifs_vm_page_mkwrite(struct vm_area_struct *vma, struct page *page)
+{
+	struct inode *inode = vma->vm_file->f_path.dentry->d_inode;
+	struct ubifs_info *c = inode->i_sb->s_fs_info;
+	struct timespec now = ubifs_current_time(inode);
+	struct ubifs_budget_req req = { .new_page = 1 };
+	int err, update_time;
+
+	dbg_gen("ino %lu, pg %lu, i_size %lld",	inode->i_ino, page->index,
+		i_size_read(inode));
+	ubifs_assert(!(inode->i_sb->s_flags & MS_RDONLY));
+
+	if (unlikely(c->ro_media))
+		return -EROFS;
+
+	/*
+	 * We have not locked @page so far so we may budget for changing the
+	 * page. Note, we cannot do this after we locked the page, because
+	 * budgeting may cause write-back which would cause deadlock.
+	 *
+	 * At the moment we do not know whether the page is dirty or not, so we
+	 * assume that it is not and budget for a new page. We could look at
+	 * the @PG_private flag and figure this out, but we may race with write
+	 * back and the page state may change by the time we lock it, so this
+	 * would need additional care. We do not bother with this at the
+	 * moment, although it might be good idea to do. Instead, we allocate
+	 * budget for a new page and amend it later on if the page was in fact
+	 * dirty.
+	 *
+	 * The budgeting-related logic of this function is similar to what we
+	 * do in 'ubifs_write_begin()' and 'ubifs_write_end()'. Glance there
+	 * for more comments.
+	 */
+	update_time = mctime_update_needed(inode, &now);
+	if (update_time)
+		/*
+		 * We have to change inode time stamp which requires extra
+		 * budgeting.
+		 */
+		req.dirtied_ino = 1;
+
+	err = ubifs_budget_space(c, &req);
+	if (unlikely(err)) {
+		if (err == -ENOSPC)
+			ubifs_warn("out of space for mmapped file "
+				   "(inode number %lu)", inode->i_ino);
+		return err;
+	}
+
+	lock_page(page);
+	if (unlikely(page->mapping != inode->i_mapping ||
+		     page_offset(page) > i_size_read(inode))) {
+		/* Page got truncated out from underneath us */
+		err = -EINVAL;
+		goto out_unlock;
+	}
+
+	if (PagePrivate(page))
+		release_new_page_budget(c);
+	else {
+		if (!PageChecked(page))
+			ubifs_convert_page_budget(c);
+		SetPagePrivate(page);
+		atomic_long_inc(&c->dirty_pg_cnt);
+		__set_page_dirty_nobuffers(page);
+	}
+
+	if (update_time) {
+		int release;
+		struct ubifs_inode *ui = ubifs_inode(inode);
+
+		mutex_lock(&ui->ui_mutex);
+		inode->i_mtime = inode->i_ctime = ubifs_current_time(inode);
+		release = ui->dirty;
+		mark_inode_dirty_sync(inode);
+		mutex_unlock(&ui->ui_mutex);
+		if (release)
+			ubifs_release_dirty_inode_budget(c, ui);
+	}
+
+	unlock_page(page);
+	return 0;
+
+out_unlock:
+	unlock_page(page);
+	ubifs_release_budget(c, &req);
+	return err;
+}
+
+static struct vm_operations_struct ubifs_file_vm_ops = {
+	.fault        = filemap_fault,
+	.page_mkwrite = ubifs_vm_page_mkwrite,
+};
+
+static int ubifs_file_mmap(struct file *file, struct vm_area_struct *vma)
+{
+	int err;
+
+	/* 'generic_file_mmap()' takes care of NOMMU case */
+	err = generic_file_mmap(file, vma);
+	if (err)
+		return err;
+	vma->vm_ops = &ubifs_file_vm_ops;
+	return 0;
+}
+
+struct address_space_operations ubifs_file_address_operations = {
+	.readpage       = ubifs_readpage,
+	.writepage      = ubifs_writepage,
+	.write_begin    = ubifs_write_begin,
+	.write_end      = ubifs_write_end,
+	.invalidatepage = ubifs_invalidatepage,
+	.set_page_dirty = ubifs_set_page_dirty,
+	.releasepage    = ubifs_releasepage,
+};
+
+struct inode_operations ubifs_file_inode_operations = {
+	.setattr     = ubifs_setattr,
+	.getattr     = ubifs_getattr,
+#ifdef CONFIG_UBIFS_FS_XATTR
+	.setxattr    = ubifs_setxattr,
+	.getxattr    = ubifs_getxattr,
+	.listxattr   = ubifs_listxattr,
+	.removexattr = ubifs_removexattr,
+#endif
+};
+
+struct inode_operations ubifs_symlink_inode_operations = {
+	.readlink    = generic_readlink,
+	.follow_link = ubifs_follow_link,
+	.setattr     = ubifs_setattr,
+	.getattr     = ubifs_getattr,
+};
+
+struct file_operations ubifs_file_operations = {
+	.llseek         = generic_file_llseek,
+	.read           = do_sync_read,
+	.write          = do_sync_write,
+	.aio_read       = generic_file_aio_read,
+	.aio_write      = ubifs_aio_write,
+	.mmap           = ubifs_file_mmap,
+	.fsync          = ubifs_fsync,
+	.unlocked_ioctl = ubifs_ioctl,
+	.splice_read	= generic_file_splice_read,
+#ifdef CONFIG_COMPAT
+	.compat_ioctl   = ubifs_compat_ioctl,
+#endif
+};